It is well known that halogenated polymers such as PVC may be heat stabilized by combinations of divalent metal carboxylates and hydrotalcite-like acid absorbers or scavengers. U.S. Pat. Nos. 4,221,687 and 5,084,499 are examples of patents disclosing such stabilizers. It is also known to stabilize vinyl halide resin compositions with mixed metal stabilizers, hydrotalcite and a dialkyltin oxide as disclosed in U.S. Pat. No. 5,561,182. Acid acceptors such as hydrotalcite and zeolite have stabilizer value in PVC and other halogenated polymers. In polymeric matrices, these additives have very little stabilizing effect and are at best secondary stabilizers.
It has been suggested that acid absorbers which are effective as stabilizers form addition complexes at degrading sites, tending to deactivate such sites and to capture catalytic, highly mobile HCI before elimination of further HCI can occur. These stabilizers are believed to have coordinating properties that enable their being at the right place at the right time, at least to a far greater extent than common, acid-reactive fillers.
When used as a primary stabilizer, additives such as hydrotalcite and zeolite (and certain others) provide stability comparable to, for example, barium stearate. In other words, process stability is long, but color hold is poor. This has been corrected in practice by combining these additives with zinc carboxylates. The effect has been to treat the acid absorber as if it were a barium or calcium component of a mixed metal stabilizer. Acid absorber particles coated with zinc carboxylates are useful in flexible PVC compositions where the application has rather modest stabilizer demand, for example, cove base extrusion. These coated acid absorbers are also costabilizers for organotin mercaptides in rigid PVC, and are widely available.
Zeolite or hydrotalcite particles coated with zinc carboxylates are relatively ineffective as costabilizers in a number of significant instances. For instance, in concert with lead stabilizers in flexible PVC, part of the lead stabilizer content may be replaced with such a costabilizer without significant loss of dynamic stability, but with no gain. Since lead stabilizers are highly cost effective, such replacement is of little value. In rigid PVC, partial replacement of lead stabilizer with zinc coated zeolite or hydrotalcite is strongly detrimental to dynamic stability. With "reverse ester" organotins, contrary to the behavior with dialkyltin thioglycolates, zinc coated zeolite or hydrotalcite is almost totally ineffective as a costabilizer with dialkyltin beta-mercaptooleates in rigid PVC compositions. Furthermore, zinc coated zeolite or hydrotalcite is largely ineffective as a costabilizer with organotin carboxylates such as dialkyltin maleates or maleate half esters. In highly filled flexible PVC compositions, such as floor tile, no advantage is found by employing zinc carboxylate coated acid absorbers.
It is believed likely that the zinc carboxylate coating of acid absorber particles functions as a phase transfer catalyst, thereby enabling close coordination to a potential degrading site. However, due to the ineffectiveness of a zinc coated zeolite or hydrotalcite in a number of instances as outlined above, alternative phase transfer catalysts or costabilizers are needed.